In order to attenuate noise, a muffler is employed and its type is generally classified as a sound absorbing type or a resonant type, etc. A sound absorbing type muffler is provided with a sound absorbing material filled within a muffler tube. Glass fiber, rock wool or mineral wool is selected for a tube filling. However, these materials cannot be used in a sound absorbing type muffler if pollution due to a sound absorbing material is to be avoided.
On the other hand, in the case of a resonant type muffler, attenuation of sound may be insufficient if the dimension of the muffler in the fluid flowing direction is such that it restricts the length of its resonant cavity. Also, in the conventional resonant type mufflers, attenuation can only be effected within only a narrow frequency band due to the provision of a single resonant cavity.